Stent

ABSTRACT

A stent and a method of making it from a wire, which method includes winding the wire on a mandrel, heating to form a coiled spring, and reversing the winding direction of the coiled spring to form the reversed coiled spring stent. The stent so formed may be reheated over a special mandrel so as to partly relax the outer portion of some or all of the stent coils. The stent may be made up of two or more sections, with adjoining section wound in opposite senses. Such a stent may be deployed by winding the stent onto a catheter, immobilizing the two ends of the wire and one or more intermediate points, bringing the stent to the location where it is to be deployed, and releasing first the intermediate point or points and then the end points. The release of the wire may be accomplished by heating the thread immobilizing the wire so that the thread breaks and releases the wire.

FIELD AND BACKGROUND OF THE INVENTION

[0001] The present invention relates to stents and, more particularly,to methods of fabricating and deploying stents.

[0002] The term “stent” has come into widespread use to denote any of alarge variety of spring-like support structures, in the form of a tubewhich is open at both ends, which can be implanted inside a blood vesselor other tubular body conduit, to help keep the vessel or conduit open.Stents may be used following balloon angioplasty to prevent restenosisand may, more generally, be used in repairing any of a number of tubularbody conduits, such as those in the vascular, biliary, genitourinary,gastrointestinal and respiratory systems, among others, which havenarrowed, weakened, distorted, distended or otherwise deformed,typically as a result of any of a number of pathological conditions.

[0003] An effective stent must possess a number of important and veryspecific characteristics. Specifically, the stent should be chemicallyand biologically inert to its surroundings and should not react with, orotherwise stimulate, the living tissues around it. The stent mustfurther be such that it will stay in the correct position and continueto support the tubular body conduit into which it is implanted overextended periods of time. Further, the stent must have the ability toreturn to its prescribed in-place diameter after the stent diameter hasbeen significantly reduced prior to its insertion, usually tightlywrapped on a catheter, into the tubular body conduit.

[0004] These requirements limit the suitable metal stent materials tojust a few metals and alloys. To date, it has been found that variousalloys of nickel and titanium (hereinafter “nitinol”), with or withoutcertain coatings, have the desired properties and are consideredsuitable for use in stent applications.

[0005] Specifically, nitinols, with or without special coatings, havebeen found to be chemically and biologically inert and to inhibitthrombus formation. Nitinols are, under certain conditions, alsosuperelastic which allows them to withstand extensive deformation andstill resume their original shape. Furthermore, nitinols possess shapememory, i.e., the metal “remembers” a specific shape fixed during aparticular heat treatment and can resort to that shape under properconditions. Shape-memory alloys can be formed into a predetermined shapeat a suitable heat treatment temperature. At temperatures below thetransition temperature range (“TTR”) certain nitinol alloys are in theirmartensite phase wherein they are highly ductile and may be plasticallydeformed into any of a number of other shapes. The alloy returns to itsaustenite phase, returning to its original predetermined shape uponreheating to a temperature above the transition temperature range. Thetransition temperature varies with each specific combination ratio ofthe components in the alloy.

[0006] The superelasticity of nitinols and their shape memorycharacteristics makes it possible to fabricate a stent having thedesired shape and dimensions. Once formed, the stent can be temporarilydeformed into a much narrower shape for insertion into the body. Once inplace, the stent can be made to resume its desired shape and dimensions.Certain alloys of nickel and titanium can be made which are plastic attemperatures below about 30° C. and are elastic at body temperatures,above 35° C. Such alloys are widely used for the production of stentsfor medical use since these nitinols are able to resume their desiredshape at normal body temperature without the need to artificially heatthe stent.

[0007] While such stents have been proven effective, they continue tosuffer from a number of disadvantages. First, there is, in certaincases, a tendency for tissue to grow in the gaps between adjoining loopsof the stent. Over time, such growth could lead to the constriction, oreven the complete closure, of the tubular body conduit in which thestent was introduced in order to keep open. A continuous, gap-free, tubestructure with no gaps would eliminate such undesired tissue growth.However, a rigid tube would lack the highly desirable flexibility whicha coiled spring configuration offers.

[0008] Another disadvantage is that the techniques for locating stentsin a body conduit are such that the stents are often installed at alocation which is not precisely the intended optimal location.

[0009] There is thus a widely recognized need for, and it would behighly advantageous to have, a stent which would be suitably flexiblebut which would significantly reduce, or even eliminate, the possibilityof undesired tissue growth between the coils of the stent.

[0010] There is further a widely recognized need for, and it would alsobe highly advantageous to have, a technique for installing stents whichwould allow the stent to be located at precisely the desired location,either by controlling the stent design or by devising adequate methodsfor its accurate release. Furthermore, in those cases where the “shapememory” characteristic is used and the stent is to be heated in itsfinal location in the body to cause it to resume its memorized shape, itis desired and advantageous to have a way of heating the stent whichsignificantly reduces, or even eliminates, the chance of damagingsurrounding tissue through heating which is conducted for too longand/or at temperatures which are too high.

SUMMARY OF THE INVENTION

[0011] According to the present invention there is provided a method offabricating a stent from a wire, comprising: (a) winding the wire on afirst mandrel; (b) heating the wound wire to form a coiled spring; and(c) after the coiled spring has cooled sufficiently, reversing thewinding direction of the coiled spring to form the stent.

[0012] Further according to the present invention there is provided astent comprising a coiled wire characterized in that the wire includesat least one section which is wound in one sense and at least onesection which is wound in the opposite sense, deployment of said stenttaking place by tightly winding the stent onto a catheter andsubsequently allowing the stent to resume its normal dimensions.

[0013] Still further according to the present invention there isprovided a method of deploying a stent in a desired location,comprising: (a) tightly winding the stent onto a catheter; (b)immobilizing at least two tie-down points on the stent using adisconnectable thread; (c) bringing the stent to the desired locationwhere the stent is to be deployed; (d) causing the thread to disconnectat one or more of the tie-down points, thereby releasing the tie-downpoint, wherein said disconnectable thread is meltable and said thread isdisconnected by heating the thread so as to cause the thread to melt.

[0014] Further yet according to the present invention there is provideda method of heating a nitinol stent to cause the stent to shift from itsmartensite phase to its austenite phase and to monitor the phase change,comprising: (a) electrically connecting the stent to an electrical powersupply; (b) supplying electrical current to the stent; (c) sensing achange in at least one electrical property to indicate the phase change;(d) controlling the current in response to the change.

[0015] The present invention successfully addresses the shortcomings ofthe presently known stents and their methods of deployment by providinga stent which is suitably flexible but which is sufficiently tight so asto eliminate the gaps between adjoining windings of the stent, therebysignificantly reducing or even eliminating the possibility ofundesirable growth of tissue between winding of the stent. The presentinvention further offers stents and associated deployment techniqueswhich make it possible to accurately install the stent in a specificlocation of a body tubular conduit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

[0017]FIG. 1 is a perspective view of a single winding of a prior artstent;

[0018]FIG. 2 is a perspective view of a single winding of stentaccording to the present invention which was obtained by reversing thewinding of a stent such as that in FIG. 1;

[0019]FIG. 3 is a side cross sectional view of a stent undergoingreheating according to the present invention, on a mandrel having twosections, each with a different heat sink capacity;

[0020]FIG. 4 is a perspective view of a stent wound and immobilized on acatheter, according to the prior art;

[0021]FIG. 5 is a close-up side cross sectional view of a portion of thesystem of FIG. 4;

[0022]FIG. 6 is a perspective view of one embodiment of a stentaccording to the present invention showing two oppositely woundsections;

[0023]FIG. 7 is a schematic side view of the stent of FIG. 6 withreference to a catheter on which the stent is delivered to its desiredlocation after release of the intermediate point;

[0024]FIG. 8 is a schematic side view of the stent of FIG. 6 when woundtightly on a catheter on which the stent is delivered to its desiredlocation;

[0025]FIG. 8A is a side view of a catheter such as might be used in FIG.8;

[0026]FIG. 8B is a side view of the catheter of FIG. 8A with the stentwound on the catheter;

[0027]FIG. 8C is a side view of the expanded stent after its release;

[0028]FIG. 9 is a perspective view of another embodiment of a stentaccording to the present invention showing a plurality of oppositelywound sections;

[0029]FIG. 10 is a schematic side view of the stent of FIG. 9 withreference to a catheter on which the stent is delivered to its desiredlocation with the stent partly released;

[0030]FIG. 11 is a schematic side view of the stent of FIG. 9 when woundtightly on a catheter on which the stent is delivered to its desiredlocation;

[0031]FIG. 12 is a perspective view of the embodiment of FIGS. 9-11showing one method of immobilizing the stent;

[0032]FIG. 13 is a close-up perspective cross sectional view of oneportion of the system of FIG. 12 showing a tie-down of an intermediatepoint;

[0033]FIG. 14 is a close-up perspective cross sectional view of oneportion of the system of FIG. 12 showing a tie-down of an end point;

[0034]FIG. 15 is a perspective view of a variation of the embodiment ofFIG. 9, showing a stent wherein the immobilization is effected insomewhat different fashion;

[0035]FIG. 15A is a side view of a catheter such as might be used inFIG. 15;

[0036]FIG. 15B is a side view of the catheter of FIG. 15A with the stentwound on the catheter;

[0037]FIG. 15C is a side view of the expanded stent as it would appearafter it has been released from the catheter;

[0038]FIG. 16 is a laid-flat view of an embodiment according to thepresent invention wherein the stent coils are encased by a film offlexible material;

[0039]FIG. 16A is a view of another embodiment of the device of FIGS.16, including an integral immobilization thread;

[0040]FIG. 16B is a side view of the device of FIG. 16A as it wouldappear when wound onto a catheter;

[0041]FIG. 17 is an end cross sectional view of the stent of FIG. 16when tightly wound onto a catheter;

[0042]FIG. 18 is a side cross sectional view of the stent of FIG. 16when tightly wound onto a catheter;

[0043]FIG. 19 is a side cross sectional view of another embodiment of astent according to the present invention when tightly wound about acatheter (not shown);

[0044]FIG. 20 is a side cross sectional view of the embodiment of FIG.19 when unwound for deployment;

[0045]FIG. 21 is a side view of stent featuring neck-down regions andtwo sections connected by a coil of low pitch;

[0046]FIG. 22 is an end cross-sectional view of the stent of FIG. 21;

[0047]FIG. 23 is a schematic cross sectional side view of a cathetershowing one embodiment of a technique for releasing the stent (notshown) using a single electrical circuit;

[0048]FIG. 24 is as in FIG. 21 except that two electrical circuits areused to provide for the sequential release of various points of thestent;

[0049]FIG. 25 is a circuit diagram for the electrical heating and phaseshift sensing of a stent according to the present invention, for theautomatic disconnection of the heating circuit upon, or at anappropriate time following, the detection of the phase shift.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] The present invention is of improved stents and of methods ofmaking and deploying them which can be used to increase theeffectiveness of stents.

[0051] The principles and operation of stents and related methodsaccording to the present invention may be better understood withreference to the drawings and the accompanying description.

[0052] Referring now to the drawing, FIG. 1 illustrates a single windingof a conventional stent. In many applications, it is important toprecisely control the flexibility of the stent as well as the interloopspacing and tightness. A number of factors must be considered inselecting the proper flexibility and interloop spacing. First, the stentmust be sufficiently flexible to follow the natural shape and dimensionsof the body conduit into which it is installed without undue stress. Thestent must also be sufficiently flexible to adequately follow thevarious movements of the conduit. These requirements tend to indicatethat a coiled, or spring-like, structure be used.

[0053] However, the stent must not be too loose since this may erode itsbody conduit support function and since when the stent loosenssignificant interloop gaps are formed which tend to encourage the growthof surrounding tissue into the separations. Such ingrowth may haveserious adverse consequences.

[0054] A stent is typically made by first tightly winding a wire of asuitable material, such as nitinol, on a mandrel. The assembly is thenheated to a suitable temperature so as to impart to the stent itsdesired shape. However, during the heating process, the mandrel is alsoheated, which brings about its expansion and leads to the formation of astent with loops which are somewhat separated from one another. Suchseparations are undesirable in certain applications.

[0055] These interloop gaps can be eliminated and the stent can bestiffened somewhat by reversing the winding direction of the stent afterit has cooled sufficiently. Shown in FIG. 2 is the single stent windingof FIG. 1 after it has been reversed. Thus, what, prior to reversal(FIG. 1), was the left end of the loop, 10, is, after reversal (FIG. 2),the right end of the loop, 10, while what, prior to reversal (FIG. 1),was the right end of the loop, 12, is, after reversal (FIG. 2), the leftend of the loop, 12. As will be appreciated, the reversal puts each loopin elastic deformation and thereby causes adjoining loops to presstogether and eliminates interloop gaps.

[0056] Under certain conditions a stent made by reversing the windingdirection as described above may be overly rigid for a specificapplication. In such a case, the rigidity of the stent may be reduced toany desired level by following a reheating procedure described below.

[0057] The reversed stent 20, whose rigidity is to be reduced, ismounted onto a mandrel 22 (FIG. 3) which may or may not be the mandrelpreviously used to give the stent its original shape. Stent 20 andmandrel 22 are reheated at a suitable temperature above the transitionpoint but the reheating is allowed to continue only long enough to allowthe outside portion of the stent (indicated in FIG. 3 as the unhatchedportion) to approach the reheating temperature and therefore to relax,while the portion of the stent near the relatively cool mandrel(indicated in FIG. 3 by hatch marks) stays at significantly lowertemperatures, does not relax, and continues to have its originalrigidity. In this way the reheated stent, upon cooling, displays aflexibility which is intermediate between those of the unreheated stentand a stent which is completely relaxed, but without opening up gapsbetween the stent loops.

[0058] The duration of the reheating must be carefully controlled toachieve the proper degree of relaxation. The reheating time will beinfluenced to a large degree by the heat properties of the mandrel. Amandrel which has high heat sink capacity, such as the left-hand portionof the mandrel of FIG. 3, can absorb considerable heat and keep thestent at low temperatures for a relatively long time.

[0059] By contrast, a mandrel which has low heat sink capacity, such asthe right-hand portion of the mandrel of FIG. 3, can absorb very littleheat and will not keep the stent at low temperatures but rather willallow that portion of the stent overlying it to quickly reach theoverall heating temperature and undergo complete relaxation.

[0060] Advantage may be taken of this property to reheat differentportions of a stent to different extents so as to achieve a finalproduct which has a certain rigidity in one or more sections and isrelaxed and features significant interloop gaps in other sections.Typically, it may be useful to have significant interloop gaps betweenthe windings near each end of the stent to facilitate the suturing ofthe stent in place.

[0061] It will be appreciated that a stent having regions of differingrelaxation characteristics can also be achieved by heating the differentsegments to different temperatures and times, such as by use of asegmented heater or furnace.

[0062] Conventional stents, as well as the reversed stents according tothe present invention described above, must be accurately placed in aspecific location in the body conduit in order to be most effective. Acommon placement technique currently used is illustrated in FIGS. 4 and5. Stent 20 is tightly wound around a catheter 24, which typicallyfeatures helical grooves 26 sized and shaped to accommodate stent 20 inits tightly wound configuration.

[0063] The two ends of stent 20 are typically bulbed, i.e., the endsfeature a slightly enlarged diameter. Each end of stent 20 isimmobilized by a thread 28 which is anchored by wrapping around catheter24 several times. Thread 28 is wrapped over the end of stent 20 as shownin FIG. 5. Catheter 24 features a small diameter bore 30 through whichruns a release wire 32. Portions of thread 28 enter transversely intobore 30 near the bulbed end of stent 20 and thread is connected withrelease wire 32 (see FIG. 5) so that as long as release wire 32 is inplace thread 28 immobilizes the end of stent 20. When both ends of stent20 are so immobilized, stent 20 is effectively prevented from unwindingand resuming its preset shape.

[0064] To deploy stent 20 in the body, catheter 24 is first brought tothe appropriate position. Release wire 32 is hen pulled, therebyreleasing the ends of stent 20. Stent 20 then immediately proceeds tounwind, enlarge and install itself into the body tubular conduit whilegetting shorter in proportion to the diameter growth, as is the case fora stent having adjoining loops which contact each other. However, in theprocess of unwinding, stent 20 assumes a final position which issomewhat arbitrary, within its original length, and which depends, tosome extent, on the local resistance encountered to the unwinding in theuneven blood vessel. The lack of certainty in the accurate placement ofthe stent often degrades its effectiveness. Hence, it is quite importantto be able to release the stent with a high degree of accuracy.

[0065] Furthermore, the unwinding action of a stent of conventionaldesign is accompanied by the rapid turning through many cycles of thestent coils. Such a turning could have a detrimental effect onsurrounding tissue since the rapid and prolonged turning could abrade orotherwise damage the interior walls of body vessels in which the stentis released.

[0066] Accordingly, a stent according to one embodiment of the presentinvention is made up of a coiled wire which is characterized in that thewire includes at least one section which is wound in one sense and atleast one section which is wound in the opposite sense. Preferably, thestent includes two sections with each of these sections representingsubstantially one half of the stent. An example of such a stent is shownin FIGS. 6-8.

[0067] Stent 120 has a central point 40 where the winding directionchanges, and two end points 42. To place stent 120 in a body conduit,stent 120 is first tightly wound onto catheter 24 and end points 42 areimmobilized using release wire 32 as described above in the context ofFIGS. 4 and 5, or in any other suitable manner. In addition, centralpoint 40 is also immobilized in a similar manner, but using a secondrelease wire 33.

[0068] To place stent 120, catheter 24 is first brought to the properlocation. Next, central point 40 is released by using second releasewire 33. This allows stent 120 to unwind without any axial displacement,since the two ends 42 are still immobilized. As stent 120 unwinds itassumes its full diameter and is firmly installed onto the inner wallsof the body tubular conduit.

[0069] At this point the two end points 42 are released by using releasewire 32, freeing stent 120 from catheter 24, and allowing the latter tobe withdrawn. Since stent 120 is already fully unwound and firmlyimplanted in the body conduit prior to the release of end points 42,stent 120 does not move upon the release of end points 42 and remainsfirmly installed in the correct position. Shown in FIGS. 8A, 8B and 8Care more detailed views of catheter 24 and stent 120 as they mightappear in an actual application.

[0070] In another embodiment of stents according to the resent inventionshowing in FIGS. 9-12, stent 220 is made up of several sections withadjoining sections wound in opposite directions. Preferably, adjoiningloops of stent 220 are wound in opposite directions, with intermediatepoints 140 representing the regions where winding directions change. Tomake such a stent, a catheter can be used which features a series ofsuitably placed pins or protrusions. The wire is wound about the mandreland use is made of the pins or protrusions to wrap the wire around thesein order to reverse the winding direction.

[0071] Shown in FIG. 12 is one scheme for attaching stent 220 tocatheter 24. Here a first release wire 132 immobilizes the two endpoints 42 and approximately one half of intermediate points 140, while asecond release wire 133 serves to immobilize the balance of intermediatepoints 140. Each of release wires 132 and 133 is preferably located inits own bore, 232 and 233, respectively. The release of such a stent isnot accompanied by the rapid and prolonged turning of the coils of thestent but is, rather, achieved by minimum and uniform turning of thecoils through approximately two turns before the stent is fully deployedin the body vessel.

[0072]FIGS. 13 and 14 depict possibilities for the actual immobilizationof an intermediate point 140 and an end point 42, respectively.

[0073] Another embodiment of a stent according to the present inventionis shown in FIG. 15, where at the end points and in the vicinity of eachwinding direction change, stent 320 features a kink or depression 50 inthe otherwise circular cross section of the stent. The kink ordepression 50 allows stent 320 to be immobilized on a catheter (notshown) by inserting a release wire (not shown) above kink or depression50 (see FIG. 15).

[0074] As can be better seen in FIGS. 15A and 15B, catheter 24preferably features slots 25 which accommodate the kinked portions ofstent 320 so that release wires 32 and 33 can pass on the outside of thekinked portions and serve to immobilizes stent 320. FIG. 15C shows stent320 as it would appear after release from catheter 24.

[0075] Other variations and improvements of methods of immobilizing andreleasing stents, whether conventional, or those according to thepresent invention, may be envisioned.

[0076] When a stent is to be inserted deep into the body, the catheterused in deploying the stent is necessarily very long and may need tofollow a highly convoluted path on its way to the desired deploymentlocation. If the stent is to be released from the catheter by pulling onthe release wire, the friction between the release wire and its bore maybe sufficiently high that pulling the release wire will result in thedeformation of the entire catheter and bring about the displacement ofthe catheter tip on which the stent is wound. This, in turn, couldresult in the improper placement of the stent.

[0077] One way of avoiding this difficulty is demonstrated in FIGS. 23and 24. Here the release wire is an electrically conducting wire which,unlike the release wires described above, is not movable but is, rather,used to conduct a small electric current upon activation by theoperator. In FIG. 23, a pair of threads 28 are shown, each of which isused to immobilize a certain point on the stent (not shown). Thread 28is in contact with a heat producing element 60 which forms a part of theelectrical circuit. Heat producing element 60 may be a resistor whichconverts electrical energy into heat. Thread 28 is made of a materialsuch that when heat producing element 60 is activated, thread 28 iscaused to melt thereby releasing the stent.

[0078] In the embodiment of FIG. 24 catheter 24 features two circuits,rather than one. This makes it possible to sequentially release variouspoints of the stent, for example, as described above. As will readily beappreciated, the basic concept can be used in a variety of related ways.For example, thread 28 can be caused to break or disconnect by cutting,by chemical reaction, and the like.

[0079] Nitinols of certain composition have transition temperaturesranges which are such that the nitinol is in its martensite phase, andis plastic, at temperatures of about 30° C. and is in its austenitephase, and highly elastic, at or above body temperatures, above about37° C. Such alloys are useful since stents made from them can be tightlywound about a catheter at room temperature and can then automaticallyresume their desired shape at normal body temperature without the needto artificially heat the stent.

[0080] However, this technique suffers from a disadvantage in that thestent may heat to body temperature prematurely, that is, before it isplaced in its intended position, and may thus suffer undesirablestresses and permanent deformation. It is, thus, useful to employnitinols which have a transition temperature range above bodytemperature (about 37° C.) such that the stent must be heated to atemperature above body temperature in order to convert the nitinol intoits austenite phase.

[0081] In such cases conventional techniques call for the heating ofstent through the circulation of hot liquids through the catheter usedto place the stent. A difficulty with such techniques is that a liquidmust be injected having a temperature which is sufficiently high so asto reach the stent at a temperature which is sufficiently high to raisethe stent temperature above the required TTR. Especially where a longcatheter must be used to reach remote body vessels, the injected liquidtemperature may be high enough to cause damage to blood and other bodytissues.

[0082] The problem is compounded by uncertainty as to when the heatingshould be discontinued, since it is difficult to know precisely when thenitinol reaches the desired temperature. As a result, there is atendency to overheat the stent, which further incurs the risk ofoverheating and damaging body tissues.

[0083] To overcome these shortcomings, it is proposed that the stent beheated electrically and that advantage be taken of the differences inthe properties of nitinols in their martensite and austenite phases tosense the change of phase of the nitinol to automatically regulate theheating.

[0084] Depicted in FIG. 25 is a circuit diagram of a stent heating andmonitoring system according to the present invention. The principles andoperation of such a system may be better understood with respect to aspecific example described next. It is to be understood that the exampleis illustrative only and does not, in any way, limit the scope of theinvention.

[0085] It is known that both the resistivity and the thermalconductivity of a nitinol alloy in its austenite phase are differentthan in its martensite phase. For example, for a particular nitinol, theresistivities are 70 and 100 μohm-cm for the martensite and austenitephases, respectively. The thermal conductivities for the same nitinolare 0.085 and 0.18 watt/cm-C° for the martensite and austenite phases,respectively.

[0086] In a system according to the present invention, stent 100 wouldbe electrically connected to a power source 102, such as a 12V battery.An appropriate first resistance 104, for example 0.018 ohm, and a secondresistance 108, for example 0.036 ohm, are provided to put a desirablevoltage drop in the martensite phase, say 7.5V, across stent 100, havingresistance of 0.09 ohm (0.5 mm diameter nd 100 mm length).

[0087] When stent 100 shifts into its austenite phase its resistancewill increase to 0.13 ohm and the voltage drop across stent 100 willincrease to 9V. The sharp change in voltage is an excellent indicationthat stent 100 has shifted into its austenite phase and can be used tocontrol the end of the heating process, either cutting off heatingimmediately upon detecting the voltage change or at a certain fixed orcalculated time thereafter.

[0088] For example, as shown in FIG. 25, the circuit can further includea transistor gate 106 with a threshold of 2.5V. As long as stent 100 isin its martensite phase the potential on transistor gate 106 will be 3Vwhich is above the threshold so that the circuit will be closed. As soonas the austenite phase is reached the potential on transistor gate 106drops to 2V, below its threshold, causing the circuit to open and theheating to be discontinued. The circuit may further have means (notshown) to continue heating beyond this point for a suitable time and ata suitable rate. It should be appreciated that a similar system can beused wherein the current drawn, rather than the voltage drop, is sensedand used to indicate the phase transition.

[0089] In some cases it is desirable that the stent form a continuouswall. This may be accomplished by encasing the wire making up the stentin a thin plastic envelop 70 (FIG. 16) which will provide the continuouswall when the stent is in position. Shown in FIGS. 17 and 18 are an endview and a side view, respectively, of stent 20 enveloped in plasticenvelop 70, as it would appear when stent 20 is tightly wound oncatheter 24.

[0090] Another embodiment of an encased stent is shown in FIGS. 16A and16B. The stent is as shown in FIG. 16 with the addition of a specialrelease loops 21, preferably made of a suitable plastic material and areconnected to plastic envelope 70 in some suitable fashion, which can beused (see FIG. 16B) to engage release wire 32 and immobilize theintermediate points of stent 20. The ends of stent 20 can be immobilizedas described above.

[0091] Yet another embodiment of an encased stent for effecting acontinuous wall upon deployment is shown in FIGS. 19 and 20. In thisembodiment a metal core 80, preferably made of nitinol, is encased in ashaped envelope 82, preferably of a suitable plastic, which allows thestent to be tightly wound on the catheter and which forms a continuoussurface when the stent is unwound. Unlike the configurations of FIGS.16-18, in the configuration of FIGS. 19 and 20, the envelope is notcontinuous and does not directly connect adjoining coils. Rather, thewire making up the stent is enveloped in a suitable material, such asplastic, which features an extension such that, when deployed, theextension serves to bridge the gap between adjoining coils of the stent.

[0092] The configuration shown in FIGS. 19 and 20 is such that when thestent expands and its metal core loops are separated from each other(FIG. 20) the stent retains its continuous sealed wall. Thus, a stent isobtained which features continuous walls and which is substantially thesame length when wound onto the catheter for delivery and placement aswhen fully deployed in the body vessel. It should be noted that such aconfiguration may be useful even without reversing of the windingdirection, since a sealed wall is maintained even when adjoining loopsare not completely contiguous.

[0093] It is to be noted that a stent according to the presentinvention, especially one featuring a continuous wall supported on ametal coil frame, as described above, is highly desirable in that such astructure is able to support the body vessel and prevent tissue ingrowthwithout undue interference with the normal flow of blood and otherbodily fluids. The latter characteristic is achieved through use of verythin coils and thin connecting walls enveloping the stent coils.

[0094] In addition, the profile and configuration of the stent can beadjusted so as to further minimize the flow friction of fluids flowinginside the stent and reduce turbulence. For example, the distal ends ofthe stent can be made to have large coil diameters than the rest of thecoils so that, when the stent is deployed, its two ends press firmlyagainst the body vessel thereby creating entrance regions for flowthrough the stent wherein the stent is essentially flush with the bodyvessel, so that drag and turbulence are minimized. It is known thatturbulence, especially in blood vessels in and near the heart, isdirectly linked to thrombus formation.

[0095] In certain applications it may be desirable for the stent tofeature uneven contour to help anchor it in place. An example is shownin FIG. 21, where depressions are placed along the coil to increase thefriction between the coil and the tissue. Furthermore, in some cases itmay be advantageous to have a stent which is made up of two sectionswhich are connected to each other by a substantially straight portion ofwire, preferably connecting points on the opposing loops of the twosections which are not corresponding points, so that the wire does notunduly press against the wall of the body vessel where there is anatural constriction in the body vessel between the two sections of thestent. Preferably the connecting wire is disposed near the periphery ofthe stent, as shown in the end cross-sectional view of FIG. 22, tominimize the obstruction to flow of fluids through the central portionsof the stent.

[0096] While the invention has been described with respect to a limitednumber of embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made. Forexample, the profile of the plastic envelope which makes up the stentcan be varied so as to better conform with the internal shape of thebody vessel wherein it is installed.

What is claimed is:
 1. A method of fabricating a stent from a wire,comprising: (a) winding the wire on a first mandrel; (b) heating thewound wire to form a coiled spring; and (c) after said coiled spring hascooled sufficiently, reversing the winding direction of said coiledspring to form the stent.
 2. A method as in claim 1, further comprising:(d) placing said stent on a second mandrel; and (e) reheating saidreversed coiled spring stent for a time sufficiently short as topartially relax said stent.
 3. A method as in claim 2 wherein saidsecond mandrel is made up of at least two regions of different heat sinkcapacities, so that the portion of said stent overlying said at leasttwo regions will be relaxed to different extents during said reheating.4. A stent made by the method of claim
 1. 5. A stent made by the methodof claim
 2. 6. A stent made by the method of claim
 3. 7. A stentcomprising a coiled wire characterized in that said wire includes atleast one section which is wound in one sense and at least one sectionwhich is wound in the opposite sense, deployment of said stent takingplace by tightly winding the stent onto a catheter and subsequentlyallowing the stent to resume its normal dimensions.
 8. A stent as inclaim 7 wherein said wire includes two sections with each of saidsections representing substantially half of said wire.
 9. A method ofdeploying a stent of claim 7 in a desired location, comprising: (a)tightly winding said stent onto a catheter; (b) immobilizing the two endpoints of said wire and at least one intermediate point on said wire;(c) bringing said stent to the desired location where said stent is tobe deployed; (d) releasing said intermediate point on the wire, therebyallowing said stent to unwind while keeping said two end pointsimmobilized; and (e) releasing said two end points of said wire.
 10. Astent as in claim 7 wherein said wire includes at least two sectionswhich are wound in one sense and at least two sections which are woundin the opposite sense.
 11. A stent as in claim 7 wherein said wireinclude a plurality of sections, each section being made up ofsubstantially a single loop.
 12. A stent as in claim 11 wherein thestent is connected to a flexible film to form a tube-like member.
 13. Amethod of deploying a stent in a desired location, comprising: (a)tightly winding said stent onto a catheter; (b) immobilizing at leasttwo tie-down points on the stent using a disconnectable thread; (c)bringing said stent to the desired location where said stent is to bedeployed; (d) causing said thread to disconnect at one or more of saidtie-down points, thereby releasing said tie-down point, wherein saiddisconnectable thread is meltable and said thread is disconnected byheating said thread so as to cause said thread to melt.
 14. A method ofheating a nitinol stent to cause the stent to shift from its martensitephase to its austenite phase and to sense the phase change, comprising:(a) electrically connecting the stent to an electrical power supply; (b)supplying electrical current to the stent; (c) sensing a change in atleast one electrical property to indicate the phase change; (d)controlling said current in response to said change.
 15. A method as inclaim 14 wherein said electrical property is a voltage drop change. 16.A method as in claim 14 wherein said electrical property is a currentchange. 17 A stent comprising a coiled wire characterized in that saidwire is enveloped in a material shaped so that when the stent is fullydeployed, portions of said envelope material occupy gaps betweenadjoining coils of the stent.